Heat removed from refluxed fluids as a ratio of withdrawn product in fractionation control



July 18. 1967 L. H. FOESTER ET AL 3,331,753

HEAT REMOVED FROM REFLUXED FLUIDS AS A RATIO OF WITHDRAWN PRODUCT IN FRACTIONATION CONTROL Filed Aug. 6, 1965 2 Sheets-Sheet 1 32 3| 33 34\ WET GAS EWATER 0 SEPARATOR Y A PRODUCT 1 5O I 4| 42 :[48 I WATER p.

' TRANSD I Lu 5 F 9 AP 3 THA 53/ v v p. Z 33 2 l 3 2 l 3 ANALOG COMPUTER /2 V v 78 Rc S.P.

19 RECYCLE- 5 80 a: TOREACTOF: v

I6 RAW m 'OIL 7 FIGURE l l5 3 RECYCLE T0 REACTOR s I R 3 H2 7 I v STEAM VAPOR FEED V I0 ll 3 7 SLURRY INVENTORS r I I4 STREAM LEWIS H. FOESTER ROBERT H. LIVESAY' I PAUL K. mass flows .A ORNEY July 18. 1967 F'QESTER- ET AL 3,331,753

HEAT REMOVED FROM REFLUXED FLUIDS AS A RATIO OF WITHDRAWN PRODUCT IN FRACTIONATION CONTROL Filed Aug. 6, 1965 2 Sheets-Sheet 2 M mflwm a T w v m" 55223. E m m u MEI F m. -mz mm w A WK S 2 mm I! u fim mm 58 Sam 2 H H 9 E k M533 I||\ m 3 R g k g 3 5a Q h 1E3 F 595 52 5:2 7 ZCBEEDm P E 8. mm r 4 8 3 H E E I -SnES E03 8 mm 1 G ESE 3 H5: 7 5555 595 52 .llllllxlllli K 02.6558 .P 2 K mm n 5- W 3 66 Sou w E N K L l 2 B F A E T G J wm 0 w S. H A V H A United States Patent Jersey Filed Aug. 6, 1965, Ser. No. 477,719 9 Claims. (Cl. 196-132) This invention relates to a fractionation control system. It is particularly applicable to the fractionation of catalytic cracking product vapors, especially vapors obtained from a so-called fluidized or fluid catalytic cracking operation.

The invention relates to a fractionation unit which is provided with means for refluxing the top of the fractionator, and means for cycling hot liquid from the upper part of the unit, by way of a heat exchanger, to the same point at which overhead condensate is returned to the unit. Cooling water is used to condense and cool the overhead vapors to provide product and reflux, and is also used, in the heat exchanger, to cool the hot liquid which is refluxed or cycled. For reasons which will become apparent hereinafter, the overhead condensate which is refluxed may be termed the separator reflux, and the hot liquid which is refluxed may be termed the recirculating reflux. The overhead condensate not refluxed (hence used as product) may be termed thefseparator product.

As previously described, heat is removed from the separator and recirculating streams by means of cooling Water. It will be appreciated that it is desirable to reduce the cooling water used for this purpose, or in other words to minimize the heat removed from these two streams, since heat given up to cooling water cannot be recovered and hence is an economic loss.

It has been found, according to this invention, that the aforementioned heat removal may be minimized, consistent with an optimum degree of fractionation in the fractionation unit, by establishing a signal representative of the sum of: 1) the ratio of the heat removed from the separator reflux stream to the flow of separator product, and (2) the ratio of the heat removed from the recirculating stream to the flow of separator prodnot; this signal is used to control the movement (flow) of some particular liquid stream, in the fractionation unit, which affects the sum of the two heat removal ratios aforementioned. The said signal is developed by an analog computer to which are fed, as inputs, signals representative of temperatures and flows in the recirculating and separator reflux streams, and of the flow in the separator product stream. In a specific aspect of the invention, the fractionation unit comprises two tower portions connected together in cascade, and what is controlled (by the signal established by, say, the computer) is the flow of liquid in the connection between these two tower portions.

An object of this invention is to provide a novel fractionation control system.

Another object is to provide a vapor fractionating means wherein the heat lost to cooling water may be minimized, while maintaining an optimum degree of fractionation.

A further object is to provide a fractionation control system wherein the total heat removed from two reflux streams which are fed into the fractionation unit at a common point is automatically maintained substantially at a minimum value.

A detailed description of the invention follows, taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic representation of a fractionation control system according to this invention; and

FIG. 2 is a schematic circuit drawing of the computer employed in the control system of FIG. 1.

The fluidized catalytic conversion of hydrocarbons is well known. In such a conversion, a hydrocarbon oil is contacted (in a reactor) with a hot fresh or regenerated catalyst mass which travels essentially as a fluid because it is fluidized with a fluidizing vapor; usually, the fluidizing vapor is the hydrocarbon to be converted. After a suitable period of contact of the hydrocarbon with the finely sub-divided fluidized catalyst particles, during which period catalytic cracking takes place, there is performed a separation, usually with the aid of a cyclone or equivalent apparatus, in which the fluidized particles are stripped of residual hydrocarbons, following which the particles are regenerated and reused. Unavoidably, some of the finely divided catalyst is entrained in the separated hydrocarbon vapors. This catalyst accumulates in a fractionation unit, in which the vapors are condensed and fractionated. The slurry of hydrocarbon liquid and finely divided catalyst is usually accumulated and removed as product or returned to the reactor. The control system of the present invention is associated with the fractionation section, which is one of the three main sections (the other two being the reactor section and the regeneration section) of a complete or overall fluid catalytic conversion apparatus.

Referring now to FIG. 1 of the drawings, a fractionation unit, indicated generally by numeral 1, comprises two tower portions, to wit, a so-called synthetic tower portion 2 and a so-called quench tower portion 3, which are connected together in cascade to operate as a single fractionation unit. The connection of these two tower portions is by way of a first pipe 4, in which vapors move from quench tower portion 3 (the lower or highertemperature portion of the fractionation unit 1) to synthetic tower portion 2 (the upper or lower-temperature portion of the fractionation unit 1), and a second pipe 5, in which liquid moves from synthetic tower portion 2 to quench tower portion 3. In actual practice, the tower portions 2 and 3 could be positioned physically side-by-side, though they act in combination as a single fractionation unit.

Hot vapor feed 6 at about 900 F., resulting from the contacting of a hydrocarbon oil such as a heavy gas oil with a cracking catalyst consisting essentially of silicaalumina, enters the bottom section of unit 1, just above the liquid therein. A bottoms or slurry draw-01f pipe 7 draws off a slurry stream from the bottom of unit 1. A portion of this stream is refluxed to a point 8 in the unit 1 above the point at which the vapor feed 6 enters, under the control of a liquid level controller 9 which actuates a valve 10 in the slurry reflux pipe 11. The hot slurry liquid from the bottom of unit 1, under the control of valve 10, is cooled in heat exchange zone 12 wherein boiler water is converted into product steam 13. That portion of the slurry stream which is not recycled or refluxed back into unit 1 (via pipe 11) is removed from the unit 1 at 14 and is either returned to the reactor (not shown), or else removed as product.

A heavy recycle stream is drawn oif from a point 15 in unit 1 above point 8. A portion of this stream is cooled in heat exchange zone 16 to which raw oil is fed for preheating, this raw oil becoming hot raw oil at 17. After passing through heat exchange zone 16, this heavy recycle stream portion is refluxed to a point 18 in unit 1 above point 15. Point 18 is near the top of quench tower portion 3. That portion of the heavy recycle stream which is not recycled or refluxed back into unit 1 (by way of heat exchange zone 16) is removed as recycle from unit 1 at 19, and is sent back to the reactor (not shown). 7

From a point 20 in unit 1 above point 18 (point 20 being located, more specifically, in the lower section of synthetic tower portion 2), a light recycle stream is 7 through heat exchange zone 21, this light recycleYstIeam portion is refluxed to a point 23 in unit 1 above point 20. That portion of the light recycle stream which is not recycledor refluxed back into unit 1 (by way of heat exchange zone 21) is removed as recycle from unit 1 at 24, and is sent back to the reactor (not shown).

From a point 25 in unit 1 above point 23, a light catalytic gas oil (LCGO) stream is drawn off. A portion of this stream is cooled in heat exchange zone 26 to which raw oil is fed for preheating, this raw oil becoming hot raw oil at 27. After passing through heat exchange zone 26, this LCGO stream portion is refluxed to a point 28 in unit 1 above point 25. That portion of the LCGO stream which is not recycled or. refluxed back into unit 1 (by way of heat exchange zone 26) is removed as product from unit 1 at 29, and is sent on to other processing units in the refinery.

Overhead vapors which are drawn oil at 30 from the top of unit 1 (which is also, of course, the top of synthetic tower portion 2), are condensed in a condenser 31 to which cooling water is supplied at 32, and are then passed to a separator 33. Wet gas is drawn off from the separator at 34 and is sent on to other processing units. The liquid condensate (which in this case comprises gasoline) is drawn off from the separator at 35. A portion of the gasoline flowing through the pipe 35 (the separator reflux) is returned 'to unit 1 as reflux, the return being made via a pipe 36 to a point 37 in unit 1 below the top thereof. The remaining portion of the gasoline flowing through pipe 35 (the separator product) is removed as product from unit 1 by pipe 38, and is sent on to other processing units.

From a point 39 in unit 1 intermediate points 37 and 23, a recirculating reflux stream (which is slightly heavier than gasoline) is drawn off under the control of a valve 40, is cooled in a heat exchanger 41 to which cooling water is supplied at 42, and is then mixed with the liquid flowing in pipe 36, for reflux or recycling back into unit 1 at 37, along with the separator reflux. The recirculating reflux is controlled by the valve 40 in response to the temperature of the overhead vapors 30 sensed through temperature controller 43. V

A thermocouple 44 senses the temperature of the overhead vapors 30 and establishes a signal T representative of the temperature of the vapor stream; signal T is fed as a first input to an analog computer 45. A thermocouple 46 senses the temperature of the separator reflux stream flowing in pipe 36 and establishes a signal T representative of the temperature of the overhead condensate used as reflux; signal T is fed as a second input to computer 45. 'An orifice-type flowmeter 47 is located in pipe 36 to sense the flow of separator reflux through this'pipe;

this flowmeter is coupled to a differential pressure has ducer 48 which produces a signal F representative of the rate of flow of separator reflux. Signal F (repre- .sentative of the rate of flow of the overhead condensate used as reflux) is fed as a third input to computer 45.

A turbine-type flowmeter 49 measures the flow of separator product in pipe 38 and produces a signal F (in the form of pulses) representative of the rate of flow of the overhead condensate not used as reflux; signal F is fed as a fourth input to computer 45.

Athermocouple 50 senses the temperature of the cooled recirculating reflux stream and establish% a .signal T representative of the temperature of this cooled liquid stream; signal T is fed as a fifth'input to computer 45. A thermocouple 51 senses the temperature of the recirculating reflux 'strearngoing into the heat exchanger 41 and establishes a signal T representative of the temperature of this hot liquid stream;-signal T is fed as. a sixth input to computer 45; An orifice-type'flowmeter 52 is located in the line between valve and heat exchanger 41 to sense the flow of recirculating reflux through this line; this flowmeter is coupled to a differential pressure transducer 53 which produces a signal F representative of the rate of flow of the (hot) recirculating reflux liquid stream. Signal F is fed as a seventh input to computer 45.

The analog computer operates to establish on its outputleads 54 a signal AH representative of the total heat removed from the two gasoline streams refluxed at 37, divided by the rate of flow of the separator product in pipe 38. That is to say, the computer output signal AH represents the sum of the heat removed from the portion of the vapor stream which is refluxed and the heat removed from the hot liquid stream taken out at point 39, both per unit volume of the condensate not used as reflux (the latter being the so-called separator produc which is the gasoline product derived from separator 33 and flowing in pipe 38). To establish this output signal AH the analog computer operates on the seven input signals previously referred to (to wit, temperatures T T T and T and flows F F and F and also takes into'account three knob-set coeflicients K K and K (constants) which will be referred to hereinafter.

The heat removed from the recirculating reflux stream is (T -T )F K where K is the specific heat of the recirculating reflux stream. The factor K is known or can be readily determined, and is assumed to remain substantially constant.

The heat removed from the separator reflux stream is 1 2) 2+ 3] 1 where K is the specific heat of the separator reflux stream, and K is the heat of vaporization of liquid on the top tray of unit 1.

The sum of the heats removed from the portion of the vapor stream which is refluxed and from the hot liquid stream, per unit volume of separator product, is given by 4 a) 2 1-r[( 1- 2) z+ a] 1 The computer 45 solves Equation 1.

Refer now to FIG. 2, wherein'computer 45 is schematically illustrated. The output electrical signal T of thermocouple 51 is applied as one of the two inputs to a substracting amplifier 55, the other input to this amplifier being the output electrical signal T of thermocouple The subtracting amplifier 55 takes the diflerence between T and T and this difference is applied as one of the two inputs to a multiplier circuit 56.

The output electrical signal from differential pressure transducer 53 appears across a resistor 57 which provides one of the inputs to the computer 45. As is well known,-

the differential pressure measurement across an orifice is proportional to the square of the flow through the conduit. Therefore, the signal across resistor 57 is applied to the input of a square root circuit 58, so that the output of this circuit is proportional to the flow F itself. The output P of circuit 58 is applied'as the other of the two inputs to multiplier circuit 56. The output of circut 56 ing across resistor 62 are integrated by a pulse rate in- V .tegrator 63, to produce a steady voltage proportional to F on output connection 64. This latter voltage is fed as the other input to divider 59, and is also fed as one input to another divider circuit 65.

Divider circuit 59 divides the product F (T T which product is the output signal of circuit 56, by F and the resulting quotient signal goes to one input of a multiplier circuit 66. The other input to circuit 66 is a knob-set adjustable voltage K which is manually set to a value representative of the specific heat of the recirculating reflux stream. The product signal output of multiplier 66 thus represents the heat removed from the recirculating reflux stream per barrel of separator product, and this signal is fed to an adder 67 The output electrical signal T of thermocouple 44 is applied as one of the two inputs to a substracting amplifier 68, the other input to this amplifier being the output electrical signal T of thermocouple 46. The subtracting amplifier 68 takes the difference between T and T and this difference is applied as one of the two inputs to a multiplier 69. The other input to multiplier 69 is a knobset adjustable voltage K (supplied for example from a suitable voltage source 70 by way of a knob-adjustable potentiometer 71), which is manually set to a value representative of the specific heat of the separator reflux stream. The output of circuit 69 represents the product of the two input signals applied thereto, and this output signal appears on output leads 72.

A knob-set adjustable voltage K is supplied in additive fashion to the leads 72, the voltage K being supplied for example from a voltage source 73 by Way of a knobadjustable potentiometer 74 and being manually set to a value representative of the heat of vaporization of liquid on the top tray of unit 1 (FIG. 1). The resulting sum (which is the output of multiplier 69 plus K is fed as one input to another multiplier 75.

The output electrical signal from differential pressure transducer 48 appears across a resistor 76, and this signal is fed through a square root circuit 77 to provide a resultant signal proportional to flow F this resultant signal is fed as the other input to multiplier 75. The output of multiplier 75 represents the product of the two input signals applied thereto, and this output is fed as one input to the divider circuit 65. The output of multiplier 751$ 1 2) 2+ s] 1- As previously stated, a voltage proportional to F (from integrator 63) is also fed as input to divider 65. Divider circuit 65 divides the product output signal of multiplier 75 by F and the resulting quotient signal is fed to adder 67. The qTiotient signal output of divider 65 represents the heat removed from the separator reflux stream per barrel of separator product.

Adder 67 adds together the two signals fed thereto, to provide an analog computer output signal AH (Equation 1) on connection 54, which signal represents the total heat removed from the two gasoline streams refluxed at 37 (FIG. 1), divided by the rate of flow of separator product.

By way of example, typical values for the various factors contained in Equation 1 will now be given. These are values typical of an actual (operating) fractionation system according to the invention. Such values are, all temperatures being in degrees Fahrenheit: temperature T 290; temperature T 85; temperature T 90; temperature T 325; flow P 2.0; flow P 6.0; flow F 7.5. Typical ranges for the constants are: K 0.5; K 0- 0.2; K 030.0. In this connection, it is noted that it is desirable to have the final result (AH the output signal of computer 45) in the form of a l050 millivolt signal, corresponding to heat removal of 40-80 M B.t.u.s per barrel; the constants in Equation 1 are scaled to take care of a thousand factor in the B.t.u.s. Also, the numbers given for flows in the example are for the flows after the square rooting which takes place in 58 and 77 (FIG. 2).

Referring back to FIG. 1, the output signal of the computer 45 is fed at 54 to a recorder-controller 78 having a set point adjustment 79. The recorder-controller 78 controls by means of a coupling schematically indicated at 80, a control valve 81 which is located in the liquiddownflow pipe '5 (between synthetic tower portion 2 and quench tower portion 3). By means of this valve, the computer output signal AH; controls (through the recorder-controller 78) the movement (flow) of liquid, in the fractionation unit 1, from a region of lower temperature (to wit, the synthetic tower portion 2) to a region of higher temperature (to wit, the quench tower portion 3). More specifically, the computer output controls the downward flow of liquid in the connection 5 between the tower portions 2 and 3. In brief, the recorder-controller 78 operates to compare the value of the computer output signal AH (representing the total heat removal in the two gasoline reflux streams, per barrel of separator product) with the value represented by the set point 79. If there is a diiference between these two values, the controller 78 operates to change the setting of valve 81, the sense of this change of setting depending upon the direction of the error of the computer output signal with respect to the set point of the controller.

The set point of controller 78 is manually adjusted to some value which has been found to be a practical minimum, consistent with good fractionation in the unit 1. If the total heat removal, per barrel of separator product, in the two gasoline reflux streams tends to increase above the desired minimum value (as a result, for example, of a change in composition of the feed to the fractionation unit, or as a result of some other change), the computer output signal AH, will of course increase accordingly (since the computer solves the heat removal equation set forth hereinabove), resulting in an output from the controller 78 which will adjust valve 81 toward the closed position. (At this juncture, it is desired to be pointed out that if the heat removal decreases below the desired minimum value, the controller 78 will adjust valve 81 toward the open position.) The flow of liquid through pipe 5, in a direction from tower section 2 to tower section 3, has the effect of a cooling of tower section 3.

' When valve 81 moves toward closed position, there will be less cooling action on tower section 3, and consequently more cooling effect on tower section 2. This means that the overhead temperature of the fractionation unit 1 will start to decrease. This decrease is sensed by temperature controller 43, resulting in a closing of valve 40 to reduce the flow P of recirculating reflux. This reduction in the flow of reflux reduces the heat removal per barrel of product, since F is in the numerator of Equation 1. Thus, the heat removal is brought back down to the desired minimum value.

Another way of explaining the foregoing action is that, when valve 81 moves toward closed position, the liquid tends to back up in synthetic tower portion 2, tending to decrease the overhead temperature of the fractionation unit 1, which results in a closing of valve 40, as previously described.

An action which is the reverse of that described above occurs when the heat removal tends to decrease below the desired minimum value; in this event, as previously stated, valve 81 will be adjusted toward the open position.

According to this invention, the computer output signal at 54 (which represents the sum of the heat removed from the two stream portions refluxed at 37, each of the two heat removals being expressed as a ratio taken per unit volume of condensate not used as reflux) is used to control the movement of liquid in some particular, selected stream in unit 1 which has the property of affecting the said sum of the two ratios, i.e., of affecting the heat removal in the total stream refluxed at 37. The heat removal effect of the stream flowing through valve 81 has previously been described. That is to say, the liquid stream flowing through pipe 5 aflects the heat removal, per barrel of product, in the total stream refluxed at 37.

Another stream in unit 1 which has the heat removal effect referred to in the preceding paragraph is that flowing from heat exchanger 21 to reflux point 23. It may be desirable, in some cases, to control this stream in response to the output signal of computer 45, rather than controlling the stream in pipe 5. To do this, the control valve 81 would be removed from pipe 5 and placed, instead,

7 in the line between heat exchanger 21 and reflux point 23.

The system of this invention operates to maintain the total heat removal from the separator reflux and the recirculating reflux streams at a minimum value; this reduces the gasoline reflux and shifts a considerable part of the heat removal to a lower section of the fractionation unit 1, making this heat recoverable in the form of generated steam or raw oil preheat, rather than being lost by being used merely to heat cooling water. However, the control system of this invention produces additional beneficial results. Since the invention operates to minimize the overhead heat removal, it reduces the liquid loading at the top of unit 1 (and this liquid loading is often a problem), it reduces the vapor loading at the top of the unit (this, also, often being a problem), and may reduce V the catalyst fines which are carried over from the fractionation unit into the various product streams.

The invention claimed is:

1. In a fractionation system wherein a feed mixture of two or more components is directed to a fractionation unit, a vapor stream is withdrawn from the top of the unit, said vapor stream is cooled to condense at least a part of same, a part of the resulting condensate is returned to said unit as reflux, a hot liquid stream is withdrawn from said unit at a point removed from the top thereof, said liquid stream is cooled, and the cooled liquid stream is mixed with said part of said condensate for reflux into said unit: a control system comprising means for establishing a signal representative of the sum of the heat removed from said refluxed part of said vapor stream and the heat removed from said hot liquid stream, each taken as a ratio of the heat removed to the flow of the condensate not used as reflux, and means responsive to said signal for controlling the flow of a selected liquid stream, in said unit, characterized in that it affects the aforementioned sum of the ratios. 7

2. System according to claim 1, wherein the first-mentioned means includes an analog computer to the input of which are fed signals representative of various temperatures and flow rates characteristic of the vapor stream, of the condensate reflux stream, of the hot liquid stream, of the cooled liquid stream, and of the stream of the condensate not used as reflux. 3. System according to claim 1, wherein the first-mentioned means includes an analog computer to the input of which are fed a first signal representative of the temperature of said vapor stream, a second signal representative of the temperature of said part of said condensate, a third signal representative of the rate of flow of said ,part of said condensate, a fourth signal representative of the rate of flow of the condensate not used as reflux, a fifth signal representative of the temperature of said cooled liquid stream, a sixth signal representative of the temperature of said hot liquid stream, and a seventh signal representative of the rate of flow of said hot liquid stream.

4. System according to claim 1, wherein the first-mentioned means includes an analog computer to the input of which are fed a first signal T representative of the temperature of said vapor stream, a second signal T representative of the temperature of said part of said condensate, a third signal F representative of the rate of flow of said part of said condensate, a fourth signal F representative of the rate of flow of the condensate not used asreflux, afifth signal T representative of the tempera- I ture of said cooled liquid stream, a sixth signal T representative of the temperature of said hot liquid stream, and a seventh signal F representative of the rate of flow of said hot liquid stream; and wherein said computer operates on the input signals fed thereto to establish an eighth signal where K K and K are constants characteristic of the compositions of the reflux streams, wherein K is the specific heat of said hot liquid stream, K is the specific heat of said condensate, and K is the heat of vaporization of said vapor stream.

5. System as defined in claim 1, wherein said unit comprises two tower portions connected together in cascade to operate as one fractionation unit, and wherein the last-mentioned means operates to control the flow of liquid in the connection between said two tower portions.

6. In a fractionation system wherein a feed mixture of two or more components is directed to a fractionation unit, a vapor stream is withdrawn from the top of the unit, said vapor stream is cooled to condense at least a part of same, a part of the resulting condensate is returned to said unit as reflux, a hot liquid stream is withdrawn from said unit at a point removed from the top thereof, said liquid stream is cooled, and the cooled liquid stream is mixed with said part of said condensate for reflux into said unit: a cont-r01 system comprising means to establish a first signal representative of the temperature of said vapor stream, means to establish a second signal representative of the temperature of said part of said condensate, means to establish a third signal representative of the rate of flow of said part of said condensate, means to establish a fourth signal representative of the rate of flow of the condensate not used as reflux, means to establish a fifth signal representative of the temperature of said cooled liquid stream, means to establish a sixth signal representative of the temperature of said hot liquid stream, means to establish a seventh signal representative of the rate of flow of said hot liquid stream, means receptive of said first through seventh signals for establishing an eighth signal representative of the sum of the heat removed from said refluxed part of said vapor stream and the heat removed from said hot liquid stream, each taken as a ratio of the heat removed to'the flow of the condensate not used as reflux, and means responsive to said eighth signal for controlling the flow of a selected liquid stream, in said unit, characterized in that it atfects the aforementioned sum of the ratios.

7. System according to claim 6, wherein said unit comprises two tower portions connected together in cascade to operate as one fractionation unit, and wherein the last-mentioned means operates to control the flow of.

turned to said unit as reflux, a hot liquid stream is with drawn from said unit at a point removed from the top thereof, said liquid stream is cooled, and the cooled liquid stream is mixed with said part of said condensate for reflux into said unit: a control system comprising means to establish a first signal T representative of the temperature of said vapor stream, means to establish a second signal T representative of the temperature of said part'of said condensate, means to establish a third signal F repre sentative of the rate of flow of said part of said condensate, means to'establish a fourth signal F representative of the rate of flow of the condensate not used as reflux, means to establish a fifth signal T representative of the temperature of said cooled liquid stream, means to establish a sixth signal T representative of the temperature of said hot liquid stream, means to establish a seventh signal F representative of the rate of flow of said hot liquid stream, means receptive of said first through seventh signals for establishing an eighth signal 4" 3) 2 1+[( 1- 2) 2+ 3] 1 3 where K K and K are constants characteristic of the eat of said Condensate, and K is the heat of vaporiza tion of said vapor stream; and means responsive to said eighth signal for controlling the flow of a selected liquid stream, in said unit, characterized in that it afiects the magnitude of said eighth signal.

9. System as defined in claim 3, wherein said unit comprises two tower portions connected together in cascade to operate as one fractionation unit, and wherein the lastmentioned means operates to control [the flow of liquid in the connection between said two tower portions.

3,143,643 8/1964 Fluegelet a1.

Wienecke 202-160 Walker 202-160 X Lupfer 196-132 X Heckart.

Marr 203-2 Walker 203-2 Lupfer et a1. 203-2 X Walker 202-160 X NORMAN YUDKOFF, Primary Examiner.

D. EDWARDS, Examiner. 

1. IN A FRACTIONATION SYSTEM WHEREIN A FEED MIXTURE OF TWO OR MORE COMPONENTS IS DIRECTED TO A FRACTIONATION UNIT, A VAPOR STRAM IS WITHDRAWN FROM THE TOP OF THE UNIT, SAID VAPOR STREAM IS COOLED TO CONDENSE AT LEAST A PART OF SAME, A PART OF THE RESULTING CONDENSATE IS RETURNED TO SAID UNIT AS REFLUX, A HOT LIQUID STREAM IS WITHDRAWN FROM SAID UNIT AT A POINT REMOVED FROM THE TOP THEREOF, SAID LIQUID STREAM IS COOLED, AND THE COOLED LIQUID STREAM IS MIXED WITH SAID PART OF SAID CONDNSATE FOR REFLUX INTO SAID UNIT; A CONTROL SYSTEM COMPRISING MEANS FOR ESTAB- 